Despite the crucial importance of estrogens in various aspects of human physioloy, such as reproduction, pregnancy and sexual development, very little is known regarding regulation of the activity and synthesis of the enzyme which makes estrogens from androgens. It is proposed in this study to purify human estrogen synthetase (aromatase) and examine various modes by which its activity and synthesis may be regulated in vitro. Because the human term placenta affords a rich and easily available source of aromatase, the study will concentrate initially on that organ. Aromatase from the microsomal fractions of term placenta will be solubilized and purified. Biochemical kinetic measurements will be perfrmed on the purified enzymes to determine the V max and K m of the various androgen substrates. The inhibitory properties of other steroidal and non-steroidal compounds will be investigated (types of inhibition and KI values). The regulation of aromatase biosynthesis will be examined in two ways. First, metabolic inhibitors of RNA and protein synthesis will be used to examine the biological level of control of aromatase activity by various stimulants (such as gonadotrophins, cyclic AMP). Also antibody to purified aromatase will be used to determine the rate of aromatase biosynthesis is measured by pulse labeling the enzyme during its synthesis with a radioactive amino acid and quantiitating the precipitated radioactiivity after the addition of aromatase antibody. These latter studies will be performed initially in the human placental systems using either long-term trophoblast tissue culture, short-term syncytiotrophoblast tissue culture, tissue slices and/or an in vitro protein synthesizing system.